1. Field of the Invention
The present invention relates to the field of electronic device fabrication including semiconductor device fabrication, and flat panel display fabrication, preferably Liquid Crystal Display (LCD) fabrication, and more particularly, to thinner compositions for selectively removing photoresist.
2. Description of the Related Art
Photoetching is used to form circuit patterns on semiconductor substrates or flat panel display substrates, for example, glass or quartz, on specific layers that have previously been formed on the substrate. Photoetching is typically carried out by coating photoresist on the substrate and causing a photo-chemical reaction on the substrate.
More particularly, a photoetching process typically involves first coating a primer over the semiconductor substrate. The primer improves the adhesiveness of the photoresist to the substrate. The photoresist is then coated onto the primer. A rinsing step is used to remove unwanted photoresist that was coated on the edges or backside of the semiconductor substrate. Subsequently, a soft baking step is used to remove solvent inside the photoresist to improve the adhesiveness of the photoresist to the semiconductor substrate. Thereafter, the photoresist is exposed and developed to create a predetermined photoresist pattern on the substrate.
The photoresist pattern formed as outlined above is used either as an etching mask for selectively etching the semiconductor substrate, as an etching mask for etching specific sublayers of the substrate, as an ion-injection mask for injecting ions into the semiconductor substrate, or as an ion-injection mask for injecting ions into a sublayer of the substrate.
Occasionally, the photoetching process results in failure. In this case, the substrate may still be saved by removing the photoresist coated on the substrate and recoating with new photoresist. This is referred to as reworking the substrate.
Chemicals known as thinners are used to remove the photoresist when reworking the substrate. The specific thinner to use differs depends on the chemistry of the substrate layer(s) below the photoresist. For example, sulfuric acid (H.sub.2 SO.sub.4), or a mixture of sulfuric acid (H.sub.2 SO.sub.4) and hydrogen peroxide (H.sub.2 O.sub.2) is used to strip photoresist when the sublayer is a non-metallic layer, while n-butyl acetate (n-BA) is used to rework the substrate when the sublayer is a metallic layer. In other words, sulfuric acid (H.sub.2 SO.sub.4) or a mixture of sulfuric acid (H.sub.2 SO.sub.4) and hydrogen peroxide (H.sub.2 O.sub.2) cannot be used to remove photoresist coated on a metallic layer, and n-butyl acetate (n-BA) cannot be used to remove photoresist coated on a non-metallic layer.
A consequence of having to use different thinners depending on the type of layer the photoresist is coated on is that a separate rework apparatus is required for each thinner. Additionally, complications can occur if the photoresist applied in the second photoetching process is incompatible with the thinner used when reworking the substrate. Conventionally, this incompatibility dictated that a different thinner or rework apparatus had to be used. Having multiple rework apparatuses decreases productivity and increases cost. Therefore, there is a need in the art for a chemical thinner that is compatible with the process of reworking the substrate and with a wide variety of photoresists.
A second problem associated with conventional thinners is that many of the conventional thinners with the necessary solubility, volatility, and viscosity for use in reworking a substrate or for use in a rinse process are toxic. Conventional thinners include ethyleneglycol monoethylether acetate (ECA), propyleneglycol monomethylether acetate (PGMEA), ethyl lactate (EL), methyl isobutyl ketone (MIBK), and n-butyl acetate (n-BA). Ethyleneglycol monoethylether acetate (ECA) has a good solubility rate, however, its volatility and flammability is unacceptably high, further, it is toxic to humans if inhaled or contacted on the skin. The toxic effects of ethyleneglycol monoethylether acetate (ECA) include leukopenia and miscarriage of an embryo. The solubility rate of propyleneglycol monomethylether acetate (PGMEA) and ethyl lactate (EL) is too low to selectively rinse photoresist from the edges or the backside of the wafer.
As previously mentioned, the rinsing step is typically carried out during manufacturing to remove any photoresist that is unnecessarily coated on the edges or the backside of the semiconductor substrate during the photoetching process. Unwanted photoresist remaining on the edges or the backside of the wafer can cause etching failure or can generate particle contamination during subsequent processing, such as during etching or ion-injection, thereby decreasing the production yield. Conventionally, in order to remove photoresist from the edges or the backside of the wafer, spray nozzles are provided above and below the wafer edge so that thinner can be sprayed onto the edges or the backside of the wafer. Thinner performance in selectively removing unwanted photoresist is determined by its solubility rate, volatility, and viscosity.
The solubility rate of a thinner determines how effectively the thinner can dissolve and remove photoresist. With respect to volatility, the thinner should easily evaporate after removing the photoresist so that it does not remain on the surface of the wafer as a potential source of pollution in subsequent processing steps. If the volatility of the thinner is too low, the thinner may remain on the edge portion of the wafers, particularly at the flat zone used in aligning the wafers after spin drying. If the thinner does not evaporate, it will build-up on the photoresist layer after development causing a lump of photoresist to form in the flat zone portion. This reduces the yield of the semiconductor devices.
Proper viscosity is essential to facilitate spraying the thinner through the nozzles. If the viscosity is too high, an excessively high spraying pressure is required. If viscosity is too low, the spray focus will deteriorate because the thinner cannot be concentrated or focused on the contact position of the wafer which interferes with the ability of the thinner to selectively remove unwanted photoresist during rinsing. Particularly, with respect to edge rinsing, the thinner must have the proper solubility rate in order to ensure a smooth wafer cross section after treatment.
Methyl isobutyl ketone (MIBK) is now widely used for rinsing photoresist, however, its high volatility can give operators headaches. Further, it can contaminate the air inside a cleanroom, cause accumulation of photoresist on the edge of the wafer because of its high volatility rate on the surface of the wafer, and cause photoresist powder to stick to the exhaust duct, thereby requiring frequent cleaning of the exhaust duct.
A rinse method using a solvent comprising an ether compound represented by the general formula of R.sup.1 --O--(--CHR.sup.2 --CH.sub.2 --O--).sub.n --H was disclosed in Korean Patent Publication No. 90-5345. According to the publication, the rinse solvent is nontoxic. This solvent may be used to remove or peel off a photoresist layer by completely immersing a wafer coated with photoresist in the solvent. However, this solvent cannot be sprayed onto a wafer to selectively remove the unwanted photoresist (i.e., remove the photoresist from the edge and backside portions of the wafer) because the viscosity is too high.
U.S. Pat. No. 4,686,280 discloses a positive-type photoresist including a trimethyl silyl nitrile used as a solvent of photoresist. Specifically, the '280 patent discloses mixing of 1,2-ethoxyl acetate, methyl ethyl ketone (MEK), xylene, and n-butyl acetate (n-BA), or use of mixtures including two or more of these chemicals, but the '280 patent does not disclose using the mixture for rinsing.
Accordingly, there remains a need for a nontoxic thinner that has the proper solubility, volatility, and viscosity rates for use in removing certain photoresists during a rework process, or for use during the rinse process for semiconductor devices. There is also a need for a reliable rework method and a method of manufacturing semiconductor devices using thinner.